it depends on your situation, if your queries need in-processing data which help doing some calculations to get the final result from the main query, and your business process is not interested in all of these data, the stored procedures will be perfect to you because retrieving those data to the BPM engine would be a waste of your resources, beside querying data is much faster inside the DB engine, and if not, direct sql query will be sufficient for you, that's for performance.
for business and logic, BPM is created to utilize business and use cases, if your statement is tightly coupled with the business and adds value to who's trying to understand it, then sql query is a way better to go, if it goes with separate logic or functionality, that doesn't directly affect the high level business, stored procedures may be achoice here.
for example if a BPM bank system is trying to query the core for customer's balance, the balance is what really matters to you, nothing else.